[codevs 1298] 凸包周长 [codevs 3201] 奶牛代理商 XI

题解：

今天开始学习计算几何。 这是一道计算几何求凸包周长的模板题，采用Andrew算法。 第二道题改下输出即可。 最后凸包周长的求法注意第一个点和最后一个点是同一个。

代码

100ms 3MB

;const int maxn = 100000 + 10;int n;struct Point {double x, y;Point(double x=0, double y=0):x(x),y(y) {}}p[maxn], ch[maxn];typedef Point Vector;Vector operator + (Vector A, Vector B) { return Vector(A.x+B.x, A.y+B.y); }Vector operator – (Vector A, Vector B) { return Vector(A.x-B.x, A.y-B.y); }Vector operator * (Vector A, double p) { return Vector(A.x*p, A.y*p); }Vector operator / (Vector A, double p) { return Vector(A.x/p, A.y/p); }bool operator < (const Vector& a, const Vector& b) {return a.x < b.x || (a.x == b.x && a.y < b.y);}const double eps = 1e-10;int dcmp(double x) {x < 0 ? -1 : 1;}bool operator == (const Vector& a, const Vector& b) {return dcmp(a.x-b.x) == 0 && dcmp(a.y-b.y) == 0;}double Dot(Vector A, Vector B) { return A.x*B.x + A.y*B.y; }double Length(Vector A) { return sqrt(Dot(A, A)); }double Angle(Vector A, Vector B) { return acos(Dot(A, B) / Length(A) / Length(B)); }double Cross(Vector A, Vector B) { return A.x*B.y – A.y*B.x; }double Area2(Point A, Point B, Point C) { return Cross(B-A, C-A); }int ConvexHull() {sort(p, p+n);int m = 0;for(int i = 0; i < n; i++) {while(m > 1 && Cross(ch[m-1]-ch[m-2], p[i]-ch[m-2]) <= 0) m–;ch[m++] = p[i];}int k = m;for(int i = n-2; i >= 0; i–) {while(m > k && Cross(ch[m-1]-ch[m-2], p[i]-ch[m-2]) <= 0) m–;ch[m++] = p[i];}if(n > 1) m–;return m;}int main() {scanf(“%d”, &n);for(int i = 0; i < n; i++) scanf(“%lf%lf”, &p[i].x, &p[i].y);int c = ConvexHull();double ans = 0;for(int i = 0; i < c; i++) ans += Length(ch[i]-ch[i+1]);printf(“%.1lf\n”, ans);return 0;}

，明天的希望，让我们忘了今天的痛苦